The function of natural killer (NK) and myeloid cells depends on the balance of a number of activating and inhibitory receptors. Natural cytotoxicity receptors (NCR) are the major receptors responsible for NK cell-mediated lysis of tumor and viral-infected cells. Recent cloning of three NCRs, NKp46, NKp44, and NKp30, prompted studies by a number of investigators to elucidate their function as well as to identify their ligands. In the past few years, we have characterized the structure and function of several inhibitory as well as activating NK cell receptors. These include the crystal structures of CD94, KIR2DL2 and its complex with HLA-Cw3, NKG2D and its complex with ULBP3, NKp46 and TREM-1. Despite the progress made in understanding the function of NK receptors in the past few years, the ligands recognized by NCR remain unknown. Our effort of understanding the function of NCR has led a structural solution for NKp30. The structure revealed striking resemblence between NKp30 and CTLA-4 family receptors and enabled us to propose a potential ligand binding site. Heavy atom derivatization is routinely used in protein structure determination and is thus of critical importance in structural biology. In order to replace the current trial-and-error heavy atom derivative screening with a knowledge-based rational derivative selection method, we systematically examined the reactivity of more than forty heavy atom compounds over a wide range of buffer and pH on peptides which contained a single reactive amino acid residue. Met, Cys and His-containing peptides were derivatized against Hg, Au and Pt compounds while Tyr, Glu, Asp, Asn, and Gln-containing peptides were assessed against Pb compounds. A total of 1668 reactive conditions were examined using mass spectrometry and compiled into heavy atom reactivity tables. The results showed that heavy atom derivatization reactions are highly linked to buffer and pH with the most accommodating buffer being MES at pH 6. A group of 21 compounds were identified as most successful irrespective of ligand or buffer/pH conditions. To assess the applicability of the peptide heavy atom reactivity to proteins, lysozyme crystals were derivatized with a list of peptide reactive compounds that included both known and new compounds for lysozyme derivatization. The results showed highly consistent heavy atom reactivities between the peptide and lysozyme.